USB-6341 and temperature measurements

[EhsanVoo]

Dear NI community,

I am trying to do electrical resistivity and temperature measurements by USB 6341 with python, separately. First of all, does USB 6341 have the capability of handling temperature by thermocouple? Since, I am doing the temperature measurements by thermocouple type E, it measures the room temperature around 150 C, which is obviously wrong.

Following is the main part the script to measure temperature:

"""

task.ai_channels.add_ai_thrmcpl_chan("Dev1/ai1",name_to_assign_to_channel="", min_val=0.0,
max_val=100.0, units=nidaqmx.constants.TemperatureUnits.DEG_C,
thermocouple_type=nidaqmx.constants.ThermocoupleType.E,
cjc_source=nidaqmx.constants.CJCSource.CONSTANT_USER_VALUE, cjc_val=150.0,
cjc_channel="")

"""

I do appreciate to hear your comments/suggestions, whether I have to change my device or any there exists any alternative script or procedure.

Best

[Jerry_X]

Is the application running in an environment where the typical temperature at USB 6341 analog front end is 150 degree C? It is returning 150 degree C because you have set the cold junction compensation (CJC) set to be 150 degree C. If the typical environment temperature is 25 degree, you can set it to be 25 degree C to get more reasonable measurements. But as the temperature of the USB 6341 analog front end changes due to its own heat dissipation or the environment, then the measurement will be less accurate over time.

If you have a signal accessory like SCB-68, you can use the CJC sensor built into the accessory: https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019MnvSAE&l=en-US

See this link to learn more about cold junction compensation: https://blog.beamex.com/thermocouple-cold-junction-compensation

[EhsanVoo]

Thanks Jerry_X,

When I repetitively measure the temperature by a loop, the values change significantly from 100 to 500 C. So, I am not quite sure this fluctuation comes from the impact of CJC.beside the dissipation problem of USB-6341, Does this instrument have the capability of measuring ordinary temperature measurements (0 -100 C) with accuracy of about 1 C?

[Jerry_X]

I originally thought it was staying stable at 150 C from the first post. If it is swinging wildly from 100 C to 500 C, then it sounds like there is physical connection problem. AI channel is 16-bit on USB-6341, and its smallest voltage range is (-0.2, 0.2) from the specification. So each code corresponds to 6 uV. For a typically type E thermocouple in temperature range 0 C to 100 C, the electrical specification seems to be fine.

To debug the possible connection issue, can we change the temperature channel to a voltage channel with range of (-0.2, 0.2) V? Looking up the raw voltage value against the specification of the thermocouple sensor should give us some ideas. Or if an accurate voltage source can be connected to same USB-6341 pins that the thermocouple is connected to, we can double check the connection by looking the voltage value returned.